D-Apiose belongs to a group of monosaccharides whose members have a branched-chain structure. These branched-chain sugars have been found in green plants, fungi, and bacteria. D-Apiose is the most common of the branched-chain sugars, being widely distributed in green plants. We have recently described the purification of an enzyme from Lemna minor which catalyzed the conversion of UDP-D-glucuronic acid to UDP-D-apiose. Enzymes or even cell-free systems involved in the synthesis of other branched-chain sugars have not been reported. One objective of this work is to understand in detail how at least one of the branched-chain sugars found in nature is biosynthesized. We plan to further purify and characterize the enzyme involved in the synthesis of UDP-D-apiose and to determine the properties of this enzyme. We particularly want to determine if the enzyme involved in UDP-D-apiose formation can be separated from the one involved in UDP-D- xylose formation. A second objective of this work is to understand the mechanism of biosynthesis of the polysaccharides present in the cell wall of L. minor, particularly the polysacharides containing D-apiose. The enzyme systems involved in their synthesis will be purified and characterized, and the products formed in vitro will be characterized and compared to the products formed in vivo. A third objective is to develop a procedure for disassembling the cell wall of L. minor, a typical green plant.